1. Field of the Invention (Technical Field)
The present invention relates primarily to the circuit topology invented by Erwin Marx in 1923; secondarily to circuit topologies that synthesize nearly rectangular, flat-topped pulses; and finally to optically triggered semiconductor switches.
The Marx Generator principle works by charging a plurality of capacitive energy storage elements (electrostatic energy stores) in parallel and discharging them in series through a plurality set of switches. Each capacitor energy store and switch set is considered to be a “stage”. Each stage can be comprised of a plurality of series and/or parallel connected energy storage elements or switches. For example, each Marx stage “switch” could be comprised of an N by M matrix of series/parallel switches. The output voltage at the terminus of the Marx Generator is typically equal to the charge voltage times the number of stages in the Marx. This circuit is analogous to putting flashlight batteries in series to increase the voltage across the bulb; however it typically works at high voltages. For example, a 100 stage Marx Generator that is charged to 10 kV per stage can produce approximately 1,000 kV at the load. Likewise, a 10 stage Marx that is charged to 100 kV per stage will produce approximately the same voltage at the load.
Marx generators also require ancillary systems for charging and triggering. The charge system consists of a power supply, power supply protection circuitry, and contain sets of charge/isolation elements that permit charging of the energy storage capacitors from a low voltage power supply (relative to the erected Marx voltage), but minimize the circulating energy losses during the pulse discharge. These elements are almost always resistors or inductors. Diodes can be used on the positive charge leg of some small, lower voltage, Marx generators, but cannot be used on the negative side as they will conduct during the discharge cycle. Resistors disipate more energy, but inductors frequently pose problems for some power supply topologies. The charge scheme can be either unipolar or bipolar. Marx generators also require a switch trigger subsystem. Spark gap systems usually trigger only the first few switches, but employ resistor chains and/or stray capacitance to assist with triggering the downstream gaps, which are naturally overvolted by the erection of the previous stages. Solid state systems require triggers for all of the switches. Electrically triggered switches must employ a high voltage isolation scheme (usually optical isolators or fiber optic cables) to protect the trigger generator during Marx erection. Optically triggered switches can be illuminated through free space, or via fiber optic cables. Because very high voltage systems require emersion in an insulating media (such as transformer oil) fiber optic bundle transport is mandatory unless diode lasers or fiber lasers are embedded with each switch. Fiber optic bundles can be illuminated by a single pulsed laser.
The series switches in conventional Marx Generators are typically gas insulated spark gaps that are constructed of two principal electrodes and usually contain a subsidiary trigger electrode.
The term modulator is derived from a modulated signal, particularly those electrical signals supplied to drive physical apparatus such as radar sets, lasers, etc. The modulator produces a pulse train that has specific amplitude, pulse shape, duration and inter-pulse period. For example, a typical modulator may produce a series of 100,000 V, one (1) microsecond long flat-top pulses at a pulse repetition rate of 100 pulses per second (Hz). A modulator typically employs pulse forming networks that tailor the pulse to obtain specific voltages, impedances, pulse durations, rise-times, fall-times, and amplitude fidelity. Although most physical apparatus require pulses that are nominally trapezoidal in shape, some of these devices can be operated with a double exponential wave shape manifest to a simple Marx generator with capacitors for energy storage elements.
2. Background Art
Note that the following discussion is given for more complete background of the scientific principles and is not to be construed as an admission that such concepts are prior art for patentability determination purposes.
Pulse power systems, such as particle beam accelerators, fusion apparatus, lasers, high power microwave systems, etc.; require high voltage electrical pulses to function properly. These requirements are currently met with different circuit topologies selected according to the desired pulse shape parameters and repetition rate specifications. Three of the most common existing techniques include simple Marx generators, where double exponential pulse shapes are acceptable; Marx generators driving pulse forming lines where short, flat-topped pulses are required; and pulse forming networks in conjunction with pulse transformers for long duration, flat-topped pulses for systems operated at modest to high repetition rates, especially for long lifetimes.
Martin Sack introduced a new Marx spark gap triggering mechanism in U.S. Pat. No. 7,170,198 B2. Sack triggers two electrode spark gaps by configuring the charge/isolation inductors to also serve as the secondary of trigger transformers. When voltage pulses are applied to the primary of the transformers, the spark gaps are overvolted and break down to initiate Marx erection. Sack mentions that the spark gap trigger generators are themselves triggered via light signals transmitted through optical fibers, but this is a standard method of achieving high voltage isolation and has no bearing on this disclosure. The Sack patent pertains exclusively to spark gap switched Marx generators and therefore has no relevance to the solid state switched Marx generators described herein.
Simple Marx generators using capacitive energy storage elements and spark gap switches have been able to satisfy many of the requirements for laboratory experiments, but are not suitable for reliable, long-life operation at high repetition rates, e.g., greater than a few tens of Hertz and a few million pulses before refurbishment. Furthermore, the simple Marx generator produces a double exponential pulse shape that is not acceptable for many applications. Finally, the spark gaps limit the system reliability, repetition rate, and lifetime between maintenance cycles.
When relatively short flat topped pulses are required, Marx generators are frequently coupled with pulse forming lines (typically oil or deionized water filled co-axial cylinders) to generate rectangular pulses on the order of one hundred nanoseconds duration. However, a water pulse forming line would have to be over fifty feet long to produce a one microsecond pulse, making it unpractical for the long pulse parameter space. Moreover, systems using Marx generators and pulse forming lines require a high voltage output switch between the pulse forming line and the load. Since no existing high voltage output switch can operate reliably at high repetition rates for long life cycles, this topology is not practical for most repetition rated devices. Marx spark gaps also limit reliability and lifetime. Finally, the additional weight, volume, and ancillary systems requisite to the deionized water pulse forming line and high voltage output switch limit the utility of such systems.
Single shot spark gap switched Marx generators have also been built by replacing the capacitors with pulse forming networks to simultaneously multiply the voltage and shape the pulse. Such Marxed-PFN's (High Energy Density Pulsers) were fabricated by the Air Force Weapons Laboratory in Albuquerque, N. Mex. in the 1960's. Twenty, Five-section, 25 kV ceramic capacitor pulse forming networks were Marxed together with spark gaps to create single shot 250 kV, 70 ns long pulses into a 70 Ohm load.
Systems that require high voltage, long duration flat-topped pulses operated at high repetition rates have been satisfied by using circuits that incorporate Pulse Forming Networks with hydrogen thyratron switches and transformers to produce shaped pulses at high voltages. However the self inductance of these transformers typically prohibits fast risetimes for such systems, and the energy lost in the rise and fall portions of the pulse lead to inefficiencies. Furthermore, such transformers are typically large and heavy and require reset circuits, all of which limits their utility.
Theodore F. Ewanizky, Jr. attempted to address the inability of the Marx generator to operate at repetition rates by substituting hydrogen thyratron switches in place of spark gaps to achieve greater pulse repetition rates and longer lifetimes; as described in U.S. Pat. No. 4,375,594: “Thyratron Marx High Voltage Generator”. However, this circuit never attained popular acceptance, probably because it is difficult to implement the heater, reservoir, and trigger circuits at the various high voltages present during the discharge of a Marx generator. Additionally, the system cost would typically be very high because of the high cost of requisite hydrogen thyratrons and the ancillary equipment and hardware required to operate the tubes. Finally, the thyratrons are typically large and difficult to mount into low inductance configurations requisite to fast rise times.
In U.S. Pat. No. 7,301,250: “High Voltage Pulsed Power Supply Using Solid State Switches”, Richard Cassel employs a Marx circuit topology to generate continuous duty, repetition rate, rectangular pulses by using capacitors that are charged in parallel and discharged in series through electrically triggered semi-conductor (solid state) switches that possess both “ON” and “OFF” capabilities. The energy stored in the Cassel circuit is much greater than that delivered to the load in a single pulse. This large capacitance increases the RC time constant, thus minimizing the pulse droop. The Cassel switches are turned on to initiate the pulse, and off to truncate the pulse at the desired pulse length. No existing photon initiated or photoconductive switches have demonstrated turn-off capabilities, and are therefore not applicable to the Cassel circuit. While sharing some similarities, the Cassel circuit differs substantially from that of the invention disclosed herein because Cassel uses a large capacitance instead of either discrete Transmission Lines or Pulse Forming Networks (that synthesize the rectangular pulses intrinsic to transmission lines) manifest to this disclosure. Furthermore, Cassel's circuit is incapable of generating rectangular pulses with closing only switches such as the photon initiated thyristors proposed herein, which do not have turnoff capabilities. Cassel makes no mention of pulse shaping via Transmission Lines or Pulse Forming Networks in lieu of the Marx stage capacitors; or of the use of “On-Only” semiconductor switches (of the closing type) such as thyristors (either electrical or optical.) Moreover, Cassel does not claim or reference optical semiconductor switches (either bulk photoconductive devices or photonically initiated multilayer devices such as the optically triggered thyristor architectures disclosed herein.)
In U.S. Pat. No. 4,900,947: “Asynchronous Marx Generator Utilizing Photo-Conductive semiconductor Switches”, Maurice Weiner, et. al. employ sequential firing of bulk small, large bandgap, photo conductive semi-conductor switches to achieve ultra-fast risetimes of less than one nanosecond into low impedance loads for small low energy transfer, insulator board mounted applications. Note reference to Levy, et al. U.S. Pat. No. 4,577,114, and to patent application Ser. No. 111,746. Weiner states that: “Typically these switches are blocks of bulk semi-insulating gallium arsenide with ohmic contacts at two ends . . . .” The carrier density of the gallium arsenide is substantially increased by irradiating it with a laser, thus creating a low resistance path between the ohmic contacts and closing the switch.” The Weiner circuit This process is effective principally because of the transmission line characteristics of the ultra-fast wave front, which inherently delays the pulse arrival time to each successive switch. The switches are triggered sequentially to coordinate with the arrival time of the transmission line wavefront. Achieving sub-nanosecond rise times into low impedance loads is a difficult task; and Weiner makes mandatory modifications to the Marx circuit (col. 1, lines 37-44), which result in crucial sacrifices to the utility of his patent. These required modifications and their consequences are discussed in the following paragraphs.
Weiner circuit modification 1: reflections must be eliminated to minimize the rise time, thereby requiring that the load resistance (Weiner FIG. 1-500) equal that of the Marx characteristic impedance (Weiner col. 4, lines 48-50), which is defined by the series inductance and the stray capacitance to the ground plane on the back side of the insulator board (Weiner col. 3, lines 48-53). The consequence of this requirement is: the output voltage (across 500) is reduced to one-half of that produced by a conventional Marx generator (given the same number of components and operating parameters). The combination of low Marx characteristic impedance and the load resistor-matching requirement severely restricts the range of applications and utility of this circuit. This restriction is significantly different than that of the invention disclosed herein because the present invention does not place any limitations on the output load resistance, except that it be significantly larger than the Marx characteristic impedance, which is the typical case. Therefore, the voltage across the load in the present circuit is nearly N*V, where N is the number of Marx stages, and V is the initial charge voltage. Hence, the versatility is greatly increased over the Weiner patent.
Weiner circuit modification 2: the backwards wave must be terminated with an input resistor (Weiner FIG. 1-600) that matches the characteristic impedance of the Marx. The consequence of this requirement is: a significant portion of the circuit energy is absorbed by the input resistor, thereby reducing the system efficacy. This restriction is significantly different than that of the invention disclosed herein because the present invention does not have an input resistor. Hence, almost all of the stored energy is deposited in the load thus maximizing the efficacy. The embodiment disclosed herein generates the same voltage and delivers the same energy to the load as the Weiner circuit with only ½ the number of capacitors and switches operated under the same conditions.
Weiner circuit modification 3: large band gap, photoconductive, bulk semiconductor switches (such as gallium arsenide) must be used to attain the required fast switching times; and the triggers must coordinate with the arrival time of the transmission line wave front. The consequence of these requirements are: the bulk photoconductive semiconductor switches identified by Weiner are intrinsically incapable of transferring large energies over long time durations, again restricting the range of potential applications. Moreover, all of the charge carriers in bulk photoconductive switches must be generated by photons in time scales less than the pulse duration, which places a high demand on the photon source. Finally, the Weiner circuit requires sequential triggering of the switches to exploit the transmission line characteristics of the Marx and thereby minimize the pulse rise time. These restrictions are significantly different than that of the invention disclosed herein because the present invention capitalizes on lower band-gap, multilayer devices (such as silicon thyristors), which can accommodate the high voltage, long duration, high current pulses requisite to numerous applications. Moreover such devices are photon initiated—but can transition to a self-sustained mode in the same manner as an electrically triggered device. Both the present disclosure and Weiner employ semiconductor Marx switches; but these switches are radically different in both architecture and performance metrics and neither can satisfy the requirements of the other. While both switch designs employ semiconductor materials and photons, they are mutually exclusive. Because the rise time of the larger embodiments disclosed herein is dominated by the inductance of the components and circuit; benefits derived from sequential triggering (to capitalize on the transmission line effect), are non-existent or inconsequential. Moreover, simultaneous triggering is probably desirable from a fault protection viewpoint.
Weiner circuit modification 4: small embodiments, e.g., insulator board mounted systems, are required to reduce the inductance to levels commensurate with sub-nanosecond rise times. This is especially true given that the load resistance must be small to match that of the Marx characteristic impedance and the exponential rise time is dominated by the Lseries/Rload time constant. The consequence of this requirement is: the Weiner circuit is incapable of storing and transferring high energy, long duration pulses because the physical size of such energy storage devices is incompatible with sub-nanosecond rise times. This restriction is significantly different than that of the invention disclosed herein because the present invention is specifically designed to store and efficiently transfer large energies to the load, which can have a wide range of resistances without affecting the utility of the circuit. Albeit, sub-nanosecond rise times are essentially impossible with the large energy configurations claimed herein because of the intrinsic inductance manifest to the typically large components and layout.
Weiner also discloses that the electrostatic energy storage elements can be comprised of Pulse Forming Lines (PFL's), which are discrete transmission lines such as strip lines, coaxial cables, etc. Short pulse duration strip lines are consistent with the fabrication of strip lines onto insulator boards, albeit, these lines must be short or the GaAs switches will fail from excessive energy transfer. Furthermore, Weiner restricts the PFL impedance to be twice that of the Marx line characteristic impedance, thereby severely limiting the range of applications and utility of the circuit (Weiner col. 4, lines 53-58). Finally, the input resistor, 600, (which is required to match the PFL impedance) absorbs ½ of the energy stored in the PFL's by terminating the backwards wave and thereby truncating the pulse duration to ½ of the pulse length of a conventional PFL. This restriction is significantly different than that of the invention disclosed herein because the present invention resides in a completely different parameter space and the PFL's have no restrictions imposed by the Marx circuit. Rather, they are designed to match the load resistance (or perveance), thereby yielding high efficacy energy transfer to the load with an optimal, nearly rectangular, pulse shape, which is not truncated. Hence, the PFL's in these two circuit topologies are radically different in design and function; and the PFL disclosed herein does not duplicate or infringe on the Weiner PFL/patent. Weiner did not anticipate this disclosure or utilization. Moreover, in this disclosure, the PFL pulse durations have very few limitations and can extend to at least a few microseconds.
Weiner makes no claim to other methods of generating rectangular pulses: including either Pulse Forming Networks (comprised of networks of discrete capacitor and inductor pairs that are configured to synthesize a transmission line); or other novel circuits comprised of inductors, resistors and switches: both of which are disclosed herein. Moreover, Weiner makes does not discuss pulse repetition rate, fault protection circuitry, tuning capabilities to optimize the pulse shape, series/parallel configured switches (or energy storage elements), parallel PFL's, or parallel Marx generators.
Weiner could not have anticipated the embodiment disclosed herein, because his circuit is simply not applicable to any purpose other than to achieve an ultra fast rise time (into low impedance loads), with short pulse durations, and low energy transfer. Adaptation to any other purpose would violate the core tenets of his patent, e.g., the requirement of matching the load resistance to the Marx characteristic impedance is highly restrictive and renders the circuit impractical for virtually all of the applications addressed in this disclosure. The required use of sequential triggering and circuit modifications, including switch topology, component selection, load and input resistor values, circuit size and layout, and energy transfer capabilities, severely restrict the utility of the Weiner patent. The Weiner circuit is totally incapable of generating a pulse consistent with a high power microwave source, Radio Frequency Linear Accelerator Klystron, or electron beam device. Nor can it be modified to serve these purposes without violating virtually every core principal of the patent.
The switches described in U.S. Pat. No. 6,154,477: “On-Board Laser-Triggered Multi-Layer Semiconductor Power Switch”, Douglas Weidenheimer, et. al., are representative of the types that may be utilized in the invention described herein.
There is a need for versatile, high energy, high average power, and long life pulse generators that are capable of generating high voltage electrical pulses with a variety of pulse shapes and durations while operating at high repetition rates. Particularly flat-topped, nearly rectangular pulses (perhaps better described as trapezoidal). Typical rise-times for these generators range from a few nanoseconds to several hundred nanoseconds; while typical pulse durations range from less than 10 nanoseconds to as much as a few milliseconds. Depending on the application, the flattopped voltage pulse may be on the order of 500 kV, with overshoot and ripple limited to values on the order of 5-10% (on the high side) to as small as 0.5% (or less) for high precision systems.